Leo H. O. Hellström

Visualizing the very-large-scale motions in turbulent pipe flow

Time-resolved stereoscopic particle image velocimetry is used to investigate the structure of the very-large-scale motions (VLSMs) in fully developed turbulent pipe flow. The motions are visualized using snapshot proper orthogonal decomposition. It is shown that the structures can be reconstructed using a small number of the most energetic modes. The results strongly suggest a possible connection between the origin of the VLSM and linear stability analysis. The structures are seen to be highly three-dimensional, meandering azimuthally and radially. At this Reynolds number (ReD=12 500), they occasionally extend from the near-wall region to the wake region of the pipe.

The energetic motions in turbulent pipe flow

Snapshot and classical proper orthogonal decomposition (POD) are used to examine the large-scale, energetic motions in fully developed turbulent pipe flow at ReD = 47,000 and 93,000. The snapshot POD modes come in pairs, representing the same azimuthal mode number but with a simple phase shift. The first 10 snapshot POD modes, associated with the very large scale motions (VLSMs), contribute 43% of the average Reynolds shear stress, and for first 80 modes u′ and v′ are anti-correlated so that they all contribute to positive shear stress events. The attached motions are contained in the lower order modes, and detached motions do not appear until snapshot POD mode numbers ≥15. We find that snapshot POD can introduce mode mixing, which is avoided in classical POD. Classical POD also gives frequency information, confirming that the low order modes capture well the behavior of the very large scale motions.

Errors in parallel-plate and cone-plate rheometer measurements due to sample underfill

The effect of sample underfill on parallel-plate and cone-plate rheometers is examined. Sample underfill can be caused by incomplete filling of a sample or loss of fluid during a test by, for example, evaporation. It is shown that even a small degree of sample underfill can lead to significant errors in measuring viscosity. A method is proposed to reduce these errors by directly monitoring the sample radius over the full course of the test. It is shown that the accuracy of the rheometer even while testing simple fluids like water is greatly improved.

Structure identification in pipe flow using proper orthogonal decomposition

The energetic motions in direct numerical simulations of turbulent pipe flow at Reτ=685 are investigated using proper orthogonal decomposition. The procedure is extended such that a pressure component is identified in addition to the three-component velocity field for each mode. The pressure component of the modes is shown to align with the streamwise velocity component associated with the large-scale motions, where positive pressure coincides with positive streamwise velocity, and vice versa. The streamwise evolution of structures is then visualized using a conditional mode, which exhibit a strong similarity to the large-scale, low-momentum motions. A low-pressure region is present in the downstream section of the structure, and a high-pressure region is present in the upstream section. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’.